The central working hypothesis of the proposed study is that the direct in vivo high throughput screening (in vivo HTS) will yield enhanced leads, thereby accelerating the discovery of analgesics with improved safety profiles. This work continues the work of the completed R21 (NIDA DA019620) entitled, In Vivo Screening of Mixture-Based Combinatorial Libraries. The overall approach described herein will accelerate drug discovery and the subsequent identification of more advanced therapeutic candidates poised for preclinical studies. The traditional approach to small molecule drug discovery is to identify individual compounds through in vitro HTS assays, before selected compounds are tested in vivo. However, such a late transition to animal testing fosters a high rate of attrition that is costly in both time and research dollars. To circumvent this problem, this proposal utilizes a novel translational approach capable of eliminating non-efficacious compounds at the earliest stage. The long-term goal of these studies is to apply in vivo HTS to accelerate drug discovery in multiple therapeutic areas. As the first step towards that goal, the specific objective in the present application applies this technique to address the need for potent, but inherently safer, analgesic compounds. Marketed opioid analgesics are both potent and effective, but are strongly addictive with potentially life- threatening side effects. This proposal will identify new chemical entities (NCEs) with the potential to advance to human clinical trials and, if successful, improve patients' quality of life while reducing the societal problems posed by nonmedical use of opioid analgesics. The potential of this proposal is clearly supportive of the mission of NIDA, to lead the Nation in bringing the power of science to bear on drug abuse and addiction. The research design uses a method of screening large, mixture-based libraries in vivo to identify compounds that are active in an in vivo mouse model of nociception. A total of 37 available, in-house, small molecule library mixtures (representing over 7 million small molecules) will be screened in vivo with animal models of antinociception to identify additional scaffolds for development, complementing a previously identified scaffold. Individual compounds selected from these three scaffolds will be synthesized and purified for additional development which will include in vitro analysis, pharmaceutical profiling, and additional in vivo models. Ultimately, we will examine multiple scaffolds and select 2-3 new chemical entities to initiate preclinical studies. By the end of the proposed study, we will have clearly demonstrated the utility of combining large mixture- based libraries of small molecules with in vivo screening (in vivo HTS) to identify therapeutic hits and leads with demonstrated efficacy and minimized side effects. By utilizing in vivo HTS, compounds may be identified with in vivo efficacies that function through previously unidentified biological pathways, providing potential advances in both the clinical and scientific realms.